1. Field of the Invention
The present invention relates to the field of single-phase A.C./A.C. converters. The function of an A.C./A.C. converter is to control an A.C. load, generally from an A.C. voltage power system, for example, the mains. The all or nothing switch, and the power variation by full halfwaves or by pulse width modulation are among the most common functions implemented by A.C./A.C. converters.
2. Discussion of the Related Art
The control of an A.C. load from the A.C. voltage power system implies the use of a switch which is bidirectional both in current and voltage (called a four-quadrant switch). The most commonly used switch is the triac.
FIG. 1 very schematically shows a conventional example of single-phase gradator of a load 1(Q) to be supplied by an A.C. voltage Vac, for example, the mains voltage. The gradator is formed of a triac 2 in series with load 1 between two terminals 3, 4, across which voltage Vac is applied. Triac 2 receives, on a control terminal, a signal CTRL which sets the turn-on delay of triac 2 with respect to the zero crossing of voltage Vac.
A disadvantage of a switch such as shown in FIG. 1 based on a triac 2 (or on an antiparallel association of two thyristors) is that this switch is not controllable to be turned off. It can thus only spontaneously turn off at the zero crossing of its current.
Another disadvantage of such a switch, which is particularly significant with an essentially resistive and/or inductive load 1, is that the delayed turning-on with respect to the zero crossing causes high reactive power consumption. Further, such a turning-on creates uncontrollable current peaks linked to the current being out of phase with the voltage.
Another disadvantage of a switch such as shown in FIG. 1 is that it is impossible to immediately order the switching off of the load, for example, after a defect has been detected. The end of the current halfwave must indeed be awaited and the next triac turning-on must not be ordered.
Several A.C./A.C. converters have already been provided to replace triacs. These solutions use one or several bicontrollable components, that is, the turning-off and the turning-on of which only occur after they are ordered. Semiconductor switches are generally used. In the present state of the art, this means using components which may be turned off in a single quadrant of their current-voltage characteristic only (for example, bipolar transistors, MOS transistors, gate turn-off thyristors, insulated gate bipolar transistors, etc.). Accordingly, the making of bicontrollable four-quadrant switching circuits is generally done by the association of at least two semiconductor components which can be triggered (turned on) and blocked (turned off) in a single quadrant only.
The fact of using turn-off controllable switches raises a problem in the case of an inductive load. Indeed, there is a risk of turning-off the switch while it is conducting a non-zero current. In this case, a high overvoltage is created across the switch, which risks deteriorating the semiconductor component by putting it in avalanche. In practice, it is seldom possible to act upon the switch control to maintain the overvoltage under its avalanche voltage due to the excessively high (or unknown) value of the load inductance.
This problem of an inductive load is raised in practice in almost all supplied loads. Indeed, no load is completely without an inductance. Accordingly, when a turn-off controllable switch is used, it is practically always necessary to have a means for pulling the load current (free wheel operation) as the switch is turned off.
FIG. 2 shows a first conventional solution for enabling the use of a turn-off controllable switching circuit operating in the four quadrants of its current-voltage characteristic, in series with a load 1.
This solution consists of placing a capacitor C in parallel with a turn-on and turn-off controllable switch 5 operating in the four quadrants.
A disadvantage of such a solution is that it requires the use of a high voltage capacitor which is thus bulky and expensive. Further, this capacitor must be able to absorb a large current upon turning-off of switch 5.
Another disadvantage of this solution is that it introduces a permanent leakage current towards the load when switch 5 is off.
Another disadvantage of the solution discussed in relation with FIG. 3 is that such a capacitor is not integrable on a semiconductor material.
FIG. 3 shows a second conventional solution to make a four-quadrant switching circuit, i.e. operating as well when the voltage thereacross is positive or negative and when the current flowing therethrough is positive or negative. This solution uses an assembly including a first four-quadrant switch 5 in series with a load 1 between two terminals 3, 4 of application of an A.C. voltage Vac, for example, the mains voltage, and a second four-quadrant switch 6 (similar to switch 5) in parallel with load 1 to form a free wheel means.
A disadvantage of such a solution is that the assembly requires four bicontrollable semiconductor switches (two per switch 5, 6) and is particularly expensive.
Another disadvantage of this solution is that it requires a complex control circuit to ensure the synchronization between the controlled turning-on of switch 5 or of switch 6 and the respective controlled turning-off of switch 6 or of switch 5.
Another disadvantage of the solution shown in FIG. 3 is that, in practice, it most often requires a circuit to protect against possible overvoltages due to a turning-off under a non-zero current in case of a defect in the control circuit.
It should be noted that, in the two conventional solutions discussed hereabove in relation with FIGS. 2 and 3, switches 5 and 6, the structure of which has not been detailed, are most often formed, each, of at least two semiconductor switches controllable to be turned off and turned on in a single quadrant of their current-voltage characteristic. This could be, for example, a parallel association of two bipolar transistors each connected in series with a diode and where the diodes are antiparallel.
The present invention aims at providing a novel solution which overcomes the disadvantages of the above-described conventional solutions.
The present invention aims, in particular, at providing a solution which creates a leakage current in the off state which can be neglected with respect to the nominal on-current, and which is, for example, entirely integrable.
The present invention also aims at providing a novel switching circuit adapted to controlling any A.C. load, with this circuit being controllable to be turned off and turned on and operating in the four quadrants of its current-voltage characteristic.
To achieve these and other objects, the present invention provides a bidirectional switching circuit including, in series, a bidirectional switching element controllable to be turned off and turned on, and a bidirectional conduction element forming a dipole and automatically selecting the conduction direction.
According to an embodiment of the present invention, the bidirectional conduction element selects the conduction direction according to the voltage across a load controlled by the switching circuit.
According to an embodiment of the present invention, the turning-on of the switching element is allowed only when the voltage across the circuit and the current in the load to be controlled have the same sign.
According to an embodiment of the present invention, the bidirectional conduction element includes, in antiparallel between two terminals, two series associations of a thyristor with a diode, the gate and the anode of each thyristor being interconnected by an avalanche diode.
According to an embodiment of the present invention, the bidirectional conduction element includes, in antiparallel between two terminals, two thyristors, the respective gates of which are short-circuited with the corresponding power terminal.
The present invention also provides a converter of an A.C. voltage into an alternating current or conversely intended for a load, using at least one switching circuit of the present invention.
According to an embodiment of the present invention, the load is connected in parallel with the bidirectional conduction element, and in series with the bidirectional switching element between two terminals of application of an A.C. voltage.
According to an embodiment of the present invention, the load is connected in parallel with the series association of the switching element and of the bidirectional conduction element, a source of alternating current being connected in parallel on the switching element.
According to an embodiment of the present invention, the load is connected between two respective midpoints of two switching circuits of the present invention.
According to an embodiment of the present invention, the bidirectional conduction element selects the conduction direction according to the voltage level thereacross and/or according to the voltage variation thereacross.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.